Gay marriage wasn't really much of an issue until its proponents made it so. They ended up shooting themselves in the foot.
As for voter turnout, is anyone going to remember this the next time we have an election? Seriously, that's enough time for two more Laci Peterson's, another three Robert Blake's, and maybe enough for Michael Jackson to touch more kids, further deform his face, and act crazy on the way to court. Oh, and maybe Paris Hilton will do something too.
The 24 hour news cycle will erase any memories of Terri Shiavo long before we have any major elections.